1. Field of the Invention
The present invention relates to an impedance converter for a microphone and a microphone. More specifically, the present invention relates to an impedance converter for a microphone and a microphone using a vacuum tube as an impedance converting element in which current flowing in a vacuum tube is controlled with a semiconductor device that performs constant current operation to enable an adjustment to achieve a balanced output.
2. Description of the Related Art
Capacitor microphones have small effective capacitance and high output impedance. Thus, for an output signal therefrom, high input impedance is required to assure frequency response at a low frequency band. Upon feeding an output signal from a capacitor microphone to an amplifier through a cable and the like, the output impedance of the capacitor microphone needs to be lowered. Therefore, capacitor microphones incorporate an impedance converter having high input impedance and low output impedance. A field-effect transistor (FET) is widely used as an impedance conversion element incorporated in a capacitor microphone.
A capacitor microphone is known that uses a vacuum tube as an impedance conversion element for higher sound quality and maximum output level (see, for example U.S. Pat. No. 6,453,048). U.S. Pat. No. 6,453,048 discloses, as an embodiment of the invention, an impedance converter including: a grounded plate amplifier tube; and a bias circuit that generates a bias voltage to be applied to the grid of the amplifier tube. The bias circuit includes: a first diode that applies a bias voltage to the grid of the amplifier tube so that a current flows to the grid; a second diode in inverse parallel connection with the first diode; and a third diode provided between the cathode of the amplifier tube and a load resistance so that a current flows from the cathode of the amplifier tube to the load resistance. With a plate current flowing in the amplifier tube, a voltage generated in the third diode is applied to the grid of the amplifier tube as a bias voltage via the first or the second diodes.
By feeding a sound signal as a result of conversion by a capacitor microphone unit to the grid of the amplifier tube, an output signal from the capacitor microphone having high input impedance can be output as a low output impedance sound signal.
The impedance converter disclosed in U.S. Pat. No. 6,453,048 outputs a signal with a triode vacuum tube in cathode follower connection because a cathode follower has high input impedance and low output impedance, and thus an increase in maximum output level can be achieved therewith.
Generally, a sound signal from a microphone is output as a balanced signal consisting of outputs from a hot side and a cold side to prevent noise attributable to an electric field or a magnetic field applied to an output transmission line of the microphone from being generated in the sound signal. To produce a balanced signal, impedances at the hot side and the cold side are required to be identical. Accordingly, an output transformer is widely used having a secondary coil with a center tap at the output circuit of the microphone to make the output impedances at the hot side and the cold side identical. Unfortunately, a transformer in an output circuit changes a tone of sound and thus may not be preferred by a user. Therefore, a microphone is available that has a circuit configuration requiring no output transformer. A conventional impedance converter used in a microphone requiring no output transformer is exemplary described below with reference to FIG. 3.
FIG. 3 illustrates microphone units 11 and 12 that each includes a diaphragm vibrated by sound pressure, and a fixed electrode facing the diaphragm with a certain space therebetween. The diaphragm of each of the microphone units 11 and 12 is connected to the ground (GRD: hereinafter referred to as the “earth”). The fixed electrode of the microphone unit 11 is connected the grid of a first vacuum tube 21 via a capacitor 15, while the fixed electrode of the microphone unit 12 is connected to the grid of a second vacuum tube 22 via a capacitor 25. In this example, the two microphone units 11 and 12 are incorporated in a single unit casing in a back-to-back manner to form a directionality variable unit. The two vacuum tubes 21 and 22 are used as impedance converters, are triodes, and are incorporated in a single glass tube to form a multiple unit tube.
A certain direct voltage needs to be applied between the diaphragm and the fixed electrode of each of the microphone units 11 and 12. In the example illustrated in FIG. 3: a negative voltage is applied to the fixed electrode of the microphone unit 11 from a direct power source 19 via a resistor 20, and diodes 13 and 14 in inverse parallel connection; while a positive voltage is applied to the fixed electrode of the microphone unit 12 from a direct power source 29 via a resistor 30, and diodes 23 and 24 in inverse parallel connection. A high direct power-supply voltage (e.g., 120 V) is applied to the plate of each of the vacuum tubes 21 and 22 from a power supply terminal 5. Via a direct voltage input terminal 4 which is a terminal in connection with the outside a power supply of 6.3 V for heating a heater 51 for the vacuum tubes 21 and 22 is supplied.
A bias circuit of each of the vacuum tubes 21 and 22 is connected as follows for cathode follower output. The cathodes of the vacuum tubes 21 and 22 are earthed via resistors 42 and 44, respectively. The cathode of the vacuum tube 21 is connected to a cold-side output terminal 3 via an electrolytic capacitor 47, while the cathode of the vacuum tube 22 is connected to a hot-side output terminal 2 via an electrolytic capacitor 48 so that a balanced output can be obtained with terminal voltages of the resistors 42 and 44. The high power-supply voltage is divided by dividing resistors 37 and 52 connected in series. In this circuit, the divided voltage is applied to the grid of the vacuum tube 21 via a resistor 38 and further via diodes 17 and 18 in inverse parallel connection. Similarly, the divided voltage is applied to the grid of the vacuum tube 22 via a resistor 39 and further via diodes 27 and 28 in inverse parallel connection. The dividing resistor 52 and the electrolytic capacitor 53 are connected in parallel. Ends of the diodes 13 and 14 in parallel connection and the diodes 17 and 18 in parallel connection are connected in parallel via coupling capacitors 15 and 16. Similarly, ends of the diodes 23 and 24 in parallel connection and the diodes 27 and 28 in parallel connection are connected in parallel via coupling capacitors 25 and 26. An electrolytic capacitor 45 is connected between a connection point between the diodes 17 and 18 and the resistor 38, and the cathode of the vacuum tube 21, while an electrolytic capacitor 46 is connected between a connection point between the diodes 27 and 28 and the resistor 39, and the cathode of the vacuum tube 22. A resistor 49 is connected between the output terminal 3 and the earth, while a resistor 50 is connected between the output terminal 2 and the earth.
As described above, or as clearly illustrated in FIG. 3, the two microphone units 11 and 12 are connected to their impedance converting elements, i.e., the vacuum tubes 21 and 22, respectively in a symmetrical manner. The bias circuit of the vacuum tube 21 includes the first diode 17 that applies a bias voltage to the grid of the corresponding vacuum tube, and the second diode 18 in inverse parallel connection with the first diode 17. The bias circuit of the vacuum tube 22 includes the first diode 27 that applies a bias voltage to the grid of the corresponding vacuum tube, and the second diode 28 in inverse parallel connection with the first diode 27. From the cathodes of the vacuum tube 21 and 22, cold side and hot side signals having opposite phases are output, respectively to achieve a balanced output. With the capacitors 15 and 16 connected on the vacuum tube 21 side, and the capacitors 25 and 26 connected on the vacuum tube 22 side, direct voltages applied to the microphone units 11 and 12 are separated from sound signals as a result of conversions by the microphone units 11 and 12. Thus, only the sound signal is fed to each of the grids of the vacuum tubes 21 and 22.
In the conventional capacitor microphone units and the impedance converters therefor shown in FIG. 3, even if the same bias voltages are applied to the two vacuum tubes 21 and 22, currents flowing in the vacuum tubes 21 and 22 are different. Obviously, variable characteristics of the two vacuum tubes lead to difference in currents. However, the currents are different even if the two vacuum tubes have stable characteristics. If the currents of the vacuum tubes 21 and 22 are different, impedances of output circuits in cathode follower connection therewith are different, resulting in an unbalanced output. Thus, if an electric field or a magnetic field is applied to the output circuit or the microphone cable, noise mixes with a sound signal.
Further, for a cathode follower output, a cathode potential must be controlled because potential difference between the cathodes of the vacuum tubes 21 and 22 and a heater tends to be large to cause insulation failure between the cathodes and the heater leading to production of noise.
The problems of a conventional capacitor microphone unit have been described. Also in the case where a ribbon microphone unit is used, a problem arises. More specifically, ribbon microphone units have extremely low output signals and thus, the output therefrom is generally boosted with a step-up transformer with an extremely large turns ratio of, for example, 1:180. Unfortunately, such a transformer with a large turns ratio makes an output impedance too high, e.g., 13 kΩ. The output impedance may be lowered with the impedance converting circuit as shown in FIG. 3, but there are the technical problems as described above.